Prior art intake ports a re typically tuned in length and in cross section area to provide either increased torque at a design speed, or a more uniform torque curve through a range of engine speeds. It is common for prior art ports to have a funneled shape, with a minimum cross section area immediately upstream of the intake valve seat(s). The port length provides resonant timings and the small port cross-section area at the outlet of the port acts as a nozzle for providing a high outlet flow velocity. The intake port is defined as the intake flow path from the intake plenum extending downstream to the intake valves.
The ratio of minimum port cross section area just up stream of the intake valve seat(s) to port cross section area at the inlet to the cylinder head (and outlet of the manifold) is typically about 1.0. A smaller port cross section area at the cylinder head inlet and extending down to the valves provides a faster flow velocity due to more nozzling of the flow, but the flow coefficient or port Cd value is generally compromised. Some ports have relatively large cross section area flowing into the cylinder head for more aerodynamic flow and a larger flow coefficient value Cd, however these ports tend to have a lower flow exit velocity due to less nozzling of the flow. The increase in cross section area improves the flow coefficient through the valves at the expense of the ram tuned exit velocity.
Venturi gaskets have been sold in the aftermarket by Spoon Sports in Japan and their distributors in the United States that significantly reduce port cross section area at the cylinder head inlet. The se venturi gaskets can modestly increase engine torque in engines that do not have very good port flow to begin with. In a typical application, torque is modestly improved in an area where torque is low to begin with, and maximum torque is no better or not significantly better than a well-designed conventional port. The venturi gaskets provide very little benefit for engines that have good port flow to begin with. With these venturi gaskets, the volume inside of the port down stream of the intake manifold is relatively small. The venturi is located between the intake manifold and cylinder head a short distance from the intake valves, and consequently the divergent nozzle is short and abrupt. The short nozzle length causes a sudden expansion of the flow and inefficient diffusion and pressure recovery.